it is probable that you received less because of a mortgage payoff. the mortgage payoff has no effect on your taxable gain just the cash you receive.
example sale price $400K mortgage payoff $100k cash received $300K
sales price $400k basis $250k capital gain $150K
however, there is a question as to your basis.
say 100% of the house was $250K
mortgage at the time of purchase $200K
net $50K
you buy 20% for $10K
your basis is the $10K + your share of the debt or 20% of $200k or $40K so your total basis is $50K
this is just an example you need to determine what your bought, what your tax basis is, and should the reduced proceeds you received the correct amount. during your ownership, did you pay your share of expenses and mortgage?
if you don't report the amount from the 1099S, expect a notice from the IRS.
the 1099-S does not report sales expenses - you need those to properly report the transaction.
